2.15.6

Schedule an accepted signal's delivery turn when another leased replay finishes waiting. Received named waits remain eligible with locally available definitions, while the existing attempted-signal and command-order fences are preserved. Fresh replay applies both signals once and leaves no extra workflow task while the timer remains pending.

Includes four timing/definition regressions and buffered/ordering controls. All five local quality commands pass, including PostgreSQL features and 77.24% unit coverage. Main qualification covers the SQL-engine, coverage and upgrade gates. The committed source-release metadata declares 2.15.6.

Refs #956, which remains open for normal Server integration and exact published Server/SDK acceptance. Core change: #957.